Cr/Sc/Mo multilayer for condenser optics in water window microscopes E C m T a b a A R R 1 A A K M S W R S 1 s a h h s o r m t o a C K c a s X u l h 0 ARTICLE IN PRESSG Model LSPEC 46622; No of Pages[.]
G Model ELSPEC-46622; No of Pages ARTICLE IN PRESS Journal of Electron Spectroscopy and Related Phenomena xxx (2016) xxx–xxx Contents lists available at ScienceDirect Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec Cr/Sc/Mo multilayer for condenser optics in water window microscopes Tadashi Hatano a,∗ , Takeo Ejima a , Toshihide Tsuru b a b IMRAM, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Faculty of Education, Art and Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan a r t i c l e i n f o Article history: Received August 2016 Received in revised form 13 December 2016 Accepted 28 December 2016 Available online xxx Keywords: Multilayer Soft X-ray Water window Reflectance Sputtering deposition a b s t r a c t We developed a high-reflectance wide-reflection-band multilayer for application in condenser optics in microscopes working in the water window soft X-ray region Grazing incidence 20 period Cr/Sc multilayers suffered damage when they were deposited on toroidal substrates, possibly because of the compressive stress of the thick Sc layers present in them To avoid such damages, we minimized the Sc layer thickness by using a Cr/Sc/Mo 10 tri-layer structure The resulting multilayer had a periodic thickness of 9.8 nm to reflect 310 eV soft X-ray at an angle of incidence of 77.2◦ The multilayers were successfully deposited on Si wafers and concave BK7 substrates by ion beam sputtering with no damage The peak height, angular acceptance, and spectral width of the measured reflectance of the Cr/Sc/Mo 10 tri-layer were 27.4%, 1.5◦ , and 35 eV, respectively, showing that it was suitable for condenser optics of broadband high-Z plasma soft X-ray sources © 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Introduction Laser produced plasmas have emerged as powerful soft X-ray sources In particular, highly charged ions of high-Z elements such as Au, Pb, and Bi radiate broadband soft X-rays [1] On the other hand, mirrors, gratings, polarizers, and other X-ray optical elements have been upgraded with multilayer coatings [2–4] In our previous study, a grazing incidence condenser mirror with a magnification of and a spot size of ∼1 mm working in the carbon window region below the K-absorption edge of C was developed [5] The mirror surface was coated with a Co/C multilayer with a period thickness of 10.9 nm This condenser mirror could be used in various types of microscopes such as the imaging- and contact-types, and in other experiments in the carbon window region Above C K-edge, microscopy in the water window region between the K-absorption edges of C and O enables the observation of living cells at a 10 nm order resolution In order to realize the practical application of Bi plasma sources for water window microscopes, shorter period grazing incidence multilayers reflecting 310 eV soft X-rays are studied Since a highly charged Bi plasma radiation is unresolved transition arrays (UTAs) and not a discrete emission line, its illumination power depends on the reflection bandwidth ∗ Corresponding author E-mail address: hatano@tagen.tohoku.ac.jp (T Hatano) of the condenser mirror as well as peak reflectance and numerical aperture The numerical aperture is 0.2 if the entire area of a 100 mm × 40 mm substrate is successfully coated [5] Therefore, the goal of the present study is to realize a high-reflectance and wide-reflection-band multilayer First, we designed and fabricated multilayers by a conventional method As described in the next section, the conventionally designed multilayers suffered from damages This led us to develop a new stacked tri-layer model in order to obtain better results Fabrication of Cr/Sc multilayers of conventional model For designing the multilayers reflecting 310 eV soft X-rays, the optical constants of several materials were plotted in Fig Sc is a quite excellent material because of its small k The small k of Cr and the large difference in n between Sc and Cr make Cr the second suitable candidate for high reflectance multilayers In normal incidence geometry, short-period high-reflectance Cr/Sc multilayers have been reported [6], and a normal incidence condenser mirror has actually been fabricated with such Cr/Sc multilayers [7], although their period number was large and consequently their reflection band was narrow Under Bragg’s condition, the period thickness of a multilayer in a grazing incidence geometry become thicker in inverse proportion to cosine of the angle of incidence Assuming a wavelength of nm and an angle of incidence of 77.2◦ , a Cr/Sc multilayer with a period thickness of 9.9 nm was obtained http://dx.doi.org/10.1016/j.elspec.2016.12.010 0368-2048/© 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Please cite this article in press as: T Hatano, et al., Cr/Sc/Mo multilayer for condenser optics in water window microscopes, J Electron Spectrosc Relat Phenom (2016), http://dx.doi.org/10.1016/j.elspec.2016.12.010 G Model ELSPEC-46622; No of Pages ARTICLE IN PRESS T Hatano et al / Journal of Electron Spectroscopy and Related Phenomena xxx (2016) xxx–xxx multilayer came off the substrate This damage is attributed to the compressive stress of the thick Sc layers present in these multilayers Design of three-material multilayer Fig Optical constants of various materials at 310 eV The layer structure was determined to be a Cr (3.7 nm)/Sc (6.2 nm) 20 bi-layer A peak reflectance of 32.2% was expected for s-polarized light when the RMS interface roughness was 0.5 nm, as shown by the dashed line in Fig First, a multilayer with this design was deposited on a Si wafer An ion beam sputtering deposition apparatus was used The reflectance was measured at the reflectometry beamline BL-11D of the Photon Factory [8,9] A satisfying result was obtained with a peak reflectance of 23.5% Then, multilayers with the same design were deposited on toroidal fused silica glass substrates In this case, just after the deposition, the multilayer coating got damaged and the damage grew in a fine lace pattern Two weeks later, the whole Fig Calculated reflectance of Cr/Sc/Mo 10 tri-layer, Cr/Sc 20 bi-layer, and Cr/Sc 10 bi-layer In addition to the stacked bi-layer model, stacked tri- or morelayer models including thin absorbing layers have been proposed to achieve high reflectance at low periodic numbers [10] For 310 eV soft X-ray, we chose Mo because the optical constants of Cr, Sc, and Mo form a large triangle, which indicates a rapid growth in the reflectance In order to determine the layer thicknesses, a layer-by-layer design method [11] was applied to the proposed stacked tri-layer model A thinner limit of 1.4 nm was assumed because an island structure appeared during the early stage of the Mo thin film growth until the thickness of the film reached 1.4 nm [12] The first Cr layer on the SiO2 or Si substrate was 5.0 nm thick The first Sc and Mo, and the second Cr layers had almost the same thickness, Sc (3.3 nm)/Mo (2.7 nm)/Cr (3.7 nm) In the subsequent tri-layers the most transparent Sc and the least transparent Mo layers became thicker and thinner, respectively From the fifth tri-layer onwards, the thicknesses of the layers became constant, and the reflectance was saturated by the tenth tri-layer The last six tri-layers were Cr (3.6 nm)/Sc (4.8 nm)/Mo (1.4 nm) and the top layer was Cr (3.3 nm) Thus, the proposed multilayer had a depth-graded distribution of individual layer thicknesses about a nominal optical period to optimize performance The total Sc layer thickness in the Cr/Sc/Mo 10 tri-layer was reduced by 63% compared to that in the Cr/Sc 20 bi-layer The calculated reflectance of the proposed Cr/Sc/Mo 10 tri-layer was 32.3% at the peak as shown by the solid line in Fig The RMS interface roughness was assumed to be 0.5 nm This peak reflectance was comparable to that of the Cr/Sc 20 bi-layer and the reflection bandwidth was larger than that of the Cr/Sc 10 bi-layer Deposition of Cr/Sc/Mo multilayers Cr (3.6 nm)/Sc (4.8 nm), Cr (3.6 nm)/Mo (1.4 nm), and Sc (4.8 nm)/Mo (1.4 nm) multilayers were deposited on Si wafers by ion beam sputtering at tentative deposition rates of Cr, Sc, and Mo All these multilayers are bi-layer stacks of the proposed tri-layer stack Their period thicknesses were measured by small angle X-ray reflectometry to correct the deposition rates Although a decrease in the layer thickness due to the formation of interfacial layers causes some error in the deposition rates, this procedure cancels out the occurrence of such errors and multilayers with desired thicknesses can be deposited A thickness controlled Cr/Sc/Mo multilayer was deposited on a Si wafer To test the multilayer deposition on concave substrates for any damage, we used a concave substrate, but substituted spherical BK7 substrates with a radius of curvature of 50 mm for the toroidal condenser substrate of R = 620 mm and = 31.7 mm [5] The position-dependent deposition rate was measured by carrying out the deposition with small Si wafers held on a cylindrical surface with a radius of curvature of 50 mm and then carrying out small angle X-ray reflectometry We deposited the Cr/Sc/Mo 10 trilayer and Cr/Sc 20 bi-layer with laterally uniform thicknesses on concave BK7 substrates using our programmable shutter system [13] No damage was observed on the surfaces of these multilayers The substrate surface was observed before and after deposition by a microscopic interferometer (VertScan, Ryoka systems Inc.) to analyze the roughness of the BK7 substrates and that of the multilayer surfaces, respectively Their RMS roughness was 1.33 and 1.12 nm, respectively It was found that the roughness of the substrate was Please cite this article in press as: T Hatano, et al., Cr/Sc/Mo multilayer for condenser optics in water window microscopes, J Electron Spectrosc Relat Phenom (2016), http://dx.doi.org/10.1016/j.elspec.2016.12.010 G Model ELSPEC-46622; No of Pages ARTICLE IN PRESS T Hatano et al / Journal of Electron Spectroscopy and Related Phenomena xxx (2016) xxx–xxx Fig Measured reflectance of the Cr/Sc/Mo 10 tri-layer deposited on a Si wafer dominant and the multilayer did not increase the roughness These roughness values will reduce the reflectance of the multilayers deposited on the concave substrate to half of that of the multilayers deposited on the flat substrates Reflectance measurements of multilayers The reflectance of the multilayers was measured at BL-11D of the Photon Factory The angular dispersion of the reflectance of a Cr/Sc/Mo 10 tri-layer on a Si wafer at 310 eV is shown in Fig The peak reflectance was 27.4% The reflection bandwidth (angular acceptance) was 1.5◦ The spectral reflectance at an angle of incidence of 77.2◦ was also measured and the spectral bandwidth was found to be 35 eV, which matches that of a UTA of highly charged Bi plasma The reflectance of the Cr/Sc/Mo 10 tri-layer and Cr/Sc 20 bi-layer deposited on concave substrates was also measured Since the elevation angle from the center to the edge become 15◦ on concave substrates with a radius of curvature of 50 mm and a diameter of substrate size of 50 mm, the 12.8◦ grazing incidence onto the center is obstructed by the edge Therefore, the reflectance measurements around an angle of incidence of 77.2◦ were carried out at a radial coordinate of r = 20 mm For this, a laterally uniform thickness deposition was required The inlets of Fig show front and side views of the reflection measurement set-up The concave substrates were rolled by 23.6◦ The results are shown in Fig The reflectance of the Cr/Sc/Mo 10 tri-layer deposited on a concave substrate was approximately 0.54 times lower than that of the Cr/Sc/Mo 10 tri-layer deposited on a flat substrate (Fig 3) This is in line with the roughness of these substrates The Cr/Sc 20 bi-layer showed a peak height same as that of the Cr/Sc/Mo 10 tri-layer and narrower angular acceptance Conclusions Fig Measured reflectance of Cr/Sc/Mo 10 tri-layer and Cr/Sc 20 bi-layer deposited on concave BK7 substrates X-ray The multilayer had a Cr/Sc/Mo 10 tri-layer structure Si wafers and concave BK7 substrates were used for the deposition of multilayers No damage was observed The reflection measurements were performed at BL-11D of the Photon Factory The measured peak reflectance of the multilayer was 27.4%, while the angular acceptance was 1.5◦ The spectral width was 35 eV, which is suitable for condenser optics of wide-band high-Z plasma sources Acknowledgments Reflectance measurements were performed under the approval of the Photon Factory Program Advisory Committee (Proposal No 2015G667) This work was partly supported by CAMS Project 2015, 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Technology for Perfect Surfaces, JSPE Publication, Tokyo, 1999, pp 292–297, Series No We designed and fabricated a multilayer reflector for grazing incidence condenser optics in microscopes operating... microscopes operating at 310 eVsoft Please cite this article in press as: T Hatano, et al., Cr/Sc/Mo multilayer for condenser optics in water window microscopes, J Electron Spectrosc Relat Phenom (2016),... substrate was Please cite this article in press as: T Hatano, et al., Cr/Sc/Mo multilayer for condenser optics in water window microscopes, J Electron Spectrosc Relat Phenom (2016), http://dx.doi.org/10.1016/j.elspec.2016.12.010